A hybrid vehicle may include an electric machine and an engine. The engine may be selectively stopped and started based on vehicle operating conditions. The electric machine may propel the hybrid vehicle when the engine has stopped rotating to conserve hydrocarbon based fuel. However, it may also be desirable at selected times to cease torque output from the electric machine and stop the electric machine from rotating while the vehicle remains activated. For example, it may be desirable to stop electric machine rotation and torque output when the hybrid vehicle is stopped for an extended period of time. The electric machine may be stopped to conserve electrical energy stored onboard the hybrid vehicle. Current flow to the electric machine may be stopped to stop the electric machine. Consequently, both hydrocarbon based fuel and stored electrical energy may be conserved during some driving conditions.
However, if the engine and the electric machine are stopped from rotating, spaces between gear surfaces within the hybrid driveline may open up. Further, application of gear clutches may not be maintained and space and/or a reduction in application force may develop between clutch friction surfaces if oil pressure in the vehicle's transmission is reduced in response to a lack of transmission pump output during the time the electric machine and engine are stopped. If the electric machine is accelerated to engine idle speed to start the engine in response to a driver demand or other conditions, the vehicle driver may notice torque being transmitted through the transmission and to vehicle wheels. In particular, the driver may notice the gear clutch engaging and impact between driveline gear teeth as the electric machine accelerates to provide the desired driver demand torque. Therefore, it would be desirable to provide a way of reducing the possibility of such driveline torque disturbances.
The inventors herein have recognized the above-mentioned disadvantages and have developed a driveline method, comprising: adjusting an electric machine from zero speed to a first speed in response to a request to propel a vehicle and driveline lash; adjusting the electric machine to a second speed after achieving the first speed in response to the request to propel the vehicle.
By rotating the electric machine at a speed that provides pressure to fill a torque converter and close gear clutches but that is slow enough to limit torque transfer through a transmission, it may be possible to provide the technical result of reducing impact between gears and reducing driveline torque disturbances. In particular, the electric machine speed may be increased from zero speed to a first speed where a transmission pump pressurizes transmission fluid to a level sufficient to begin closing gear clutches and that allows torque to be transmitted from the torque converter impeller to the torque converter turbine. The first speed is may be a speed where the torque converter efficiency is less than a threshold efficiency or a speed where the torque converter transmits less than a threshold amount of torque. In this way, a small amount of torque may be transferred through the transmission to reduce gaps between meshing surfaces before an amount of torque sufficient to move the vehicle is transferred through the driveline. Consequently, the possibility of driveline torque disturbances may be reduced.
The present description may provide several advantages. For example, the approach may reduce driveline torque disturbances. Additionally, the approach may reduce wear of driveline components. Further, the approach may improve acceptance of stopping the electric machine to improve vehicle efficiency.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.